This invention generally relates to the manufacture of flexible printed circuit boards (hereinafter “flex circuits”). More specifically, the invention relates to the use of frames for processing flex circuits.
Current flex circuit manufacturing processes utilize roll-to-roll or large-panel fabrication equipment. For extremely high volume, roll-to-roll processing is perceived to be the lowest-cost approach.
However, the current state-of-the-art roll-to-roll processing requires 2-mil-thick polyimide film to maintain dimensional stability through the via formation, metal deposition and patterning steps. For double-sided flex circuitry and high-end flex circuitry, certain applications will require thinner polyimide, ultimately as thin as ½ mil. Current roll-to-roll production lines cannot handle this thickness of polyimide because of distortion issues during processing, and also because of alignment tolerances required for registration of double-sided flex circuitry with micro vias.
Organic polymeric materials such as polyimides have physical and electrical properties that are extremely susceptible to their environment, whether it be humidity, temperature, or processing history. Polyimide films in general have a large coefficient of thermal expansion (CTE) and a large coefficient of humidity expansion (CHE). Depending on the molecular structure, these values can vary significantly from one polymer to another. The properties of some polyimide films used in flex circuit production are summarized in Table1 (see Appendix).
The Kapton® polyimides are a product supplied by DuPont Microelectronics, Wilmington, Del., and the Upilex® polyimides are commercially available from Ube Chemicals, Japan. These polymers are manufactured on a roll-to-roll line and are supplied as thin films ranging in thickness from 0.5 to 5 mils. The physical properties may vary slightly from batch to batch depending on the exact nature of the manufacturing history. During flex circuit production, the polyimide film is exposed to varying temperature and humidity conditions that may cause expansion and shrinkage both in the plane of the film and perpendicular to the plane of the film. These temperature and humidity excursions result in pattern distortions that limit the size of features that can be patterned in high yield. To minimize these distortion issues, manufacturers have utilized 2-mil-thick polyimide film. Thicker polyimide can be handled in high-volume roll-to-roll manufacturing, whereas thinner polyimide films are more susceptible to heat and mechanical distortion in high-temperature bake and vacuum deposition steps.
Current panel processing relies on mechanical pins over which the flex film is registered during processing. Although it is possible to fabricate high-resolution structures over very small areas using this approach, the tolerance budget for double-sided fine-line flex circuitry makes this approach unmanufacturable. Other high-end flex circuit manufacturers resort to bonding the entire film to a rigid substrate during processing, and releasing the film upon completion. This approach requires additional processing, prevents the advantage of double-sided processing, induces stresses in the polyimide, and adds defects from the bonding material.
U.S. Pat. No. 6,323,096 discloses a method for fabricating a flexible interconnect film that involves using a rigid frame to support a dielectric film during processing. There is a continuing need for improved methods of fabricating flex circuits using rigid frames to support the workpiece and minimize distortion during processing.